1. Field of the Invention
This invention relates to an encrypting and decrypting system for encrypting an incoming analog signal into an encrypted analog signal and for decrypting the encrypted analog signal into an decrypted analog signal which is analogous to the incoming signal.
More particularly, the invention concerns the encrypting and decrypting of an audiofrequency signal of a radiophonic program or, more generally, to the encoding and decoding, or scrambling and unscrambling, or enciphering and deciphering of an analog signal, thereby obtaining a non-intelligible signal to be transmitted from a station transmitter to listener receivers.
2. Description of the Prior Art
When a radio or television station wishes to broadcast a specific subject directed at a well-defined category of listeners, this program generally has to be broadcast at night, namely outside the public's peak listening hours. Since some listeners do not listen at night, automatic recording receivers may be provided which use a magnetophone or magnetoscope, to permit the hours of reception to be virtually independent of the listeners' program listening time.
However, when a specialized program is heard solely by a specialized class, such as doctors in the case of a medical program it may be undesirable for other people to listen in. In this case, the listeners are selected from encrypting the audiofrequency signal broadcast by the radio-communications or television transmitter in accordance with a "key" or encrypting code and by thereafter decrypting the audiofrequency signal picked up by the listener's receiver in accordance with the "key" or decrypting code corresponding to the inverse operation of the encrypting code operation. These encrypting and decrypting operations are made applicable to analog signals such as speech and musical signals.
Known encrypting and decrypting systems in the prior art implement an analog sampling of the incoming analog signal at periodic time intervals at predetermined instants. Thereafter there takes place a scrambling of the analog samples. Known methods of arithmetic encoding can be applied, the simplest ones consisting of an encoding in accordance with a pseudo-random sequence or with permutation sequences of two or several analog samples. An example is found in U.S. Pat. No. 4,100,374.
The U.S. Pat. No. 4,100,374 discloses an encrypting and decrypting system based on an analog sample permutation method. The delay means which is included in the encrypter (or the decrypter) of this patent comprises two analog shift registers each having N stages. The inputs of the first stages of the two shift registers receive N samples of the incoming signal to be encrypted (or the encrypted signal). Each of the two shift registers time delays of N incoming signal samples (or N encrypted signal samples) during every other period NT.sub.W of the encrypting signal (or the decrypting signal). The 2 N stage outputs of the two shift registers are connected to the output of the encrypter (or the decrypter) through an analog switch which is analogous to a parallel-to-series converter. The analog switch is controlled by the encrypting signal (or the decrypting signal) so as to select the N outputs of one of shift registers then the N outputs of the other shift register during two consecutive periods NT.sub.W. During each time NT.sub.W, the N outputs of a shift register are selected according to a predetermined encrypting sequence so as to transpose and read the previously written samples according to a various arrangement. This is equivalent to a permutation of samples which is synchronized at a reading frequency equal to the writing frequency 1/T.sub.W. The encrypting signal controls also the addressing of the N outputs of a shift register according to a predetermined permutation and at a constant reading frequency.
For reconstructing the initial signal in the decrypter, the decrypting signal is composed of a series of stage addressing words in according with the complementary permutation to the encrypting permutation. Consequently, the encrypting signal producing means and the decrypting signal producing means are both necessary. In addition, the fact that the addressing order of the outputs of shift registers according to a predetermined permutation differs from the initial order of the received signal samples, complicates the logic circuitry of the system. As a result of this arrangement, the cost of the system is relatively high, so that the number of the listeners who can afford a decrypter for specialized programs is reduced.